Hydraulic power units such as, for example, accumulator apparatus, are often employed in hydraulic systems to provide, for example, energy storage, fluid compensation, energy accumulation, pulsation damping, etc. For example, when employed as energy storage units, accumulator apparatus may be used to provide pressurized control fluid (e.g., hydraulic oil) to equipment (e.g., hydraulic equipment) such as cylinders, valve actuators, or other machinery requiring high pressure fluid to operate. For example, an accumulator may be used to store pressurized hydraulic fluid provided by a hydraulic pump when the hydraulic system demand is low (e.g., a hydraulic actuator is not being actuated) and to supply the previously stored pressurized hydraulic fluid to the system to provide additional energy when the demand of the hydraulic system increases (e.g., the hydraulic actuator is being actuated).
Accumulator apparatus such as, for example, hydraulic accumulator apparatus typically include a housing or cylinder having two chambers separated by a piston. A first chamber may be fluidly coupled to a hydraulic system to receive pressurized hydraulic fluid. A second chamber is typically filled or pre-charged or, more generally, charged with an inert gas such as, for example, a dry nitrogen gas. A seal surrounds the piston to prevent leakage of the hydraulic fluid and/or the inert gas across the piston between the first and second chambers.
In operation, pressurized hydraulic fluid is stored in the first chamber via a pump. The hydraulic fluid acts on a first side of the piston via the first chamber to cause the piston to move toward the second chamber to a stored position. As the piston moves toward the stored position, the volume of the second chamber is reduced, thereby compressing the gas in the second chamber. As a result, the pressure of the gas in the second chamber increases until a force exerted on the first side of the piston by the pressure of the hydraulic fluid in the first chamber is substantially equal to a force exerted on a second side of the piston by the pressure of the compressed gas in the second chamber. During operation, accumulators can remain in the stored position for a relatively long period of time. Thus, the gas in the second chamber may be subjected to high pressure levels for a relatively long period of time.
When the demand of the hydraulic system increases, the pressure of the hydraulic fluid in the first chamber decreases. When the pressure of the hydraulic fluid decreases below the pressure of the compressed gas, the gas expands and drives the piston toward the first chamber and exerts a force on the hydraulic fluid via the piston. As a result, the accumulator apparatus supplies the hydraulic system with previously stored pressurized hydraulic fluid. The pre-charged pressure of the gas in the second chamber determines the minimum system pressure provided by the accumulator apparatus.
Some known accumulator apparatus have a housing that includes a pre-charge port or connection (e.g., a threaded port, a threaded connector) fluidly coupled to the second chamber to pre-charge or charge the accumulator apparatus. An inert gas such as a dry nitrogen gas may be supplied from a tank or vessel to the second chamber via the pre-charge port or connection. However, the gas may leak slowly from the second chamber to the environment via the pre-charge port or connection. For example, pre-charge ports or connections of some known accumulator apparatus exposed to relatively high vibration environments may loosen and cause leakage of the gas. Such leakage typically occurs when the piston is at the stored position because the pressure of the gas is relatively high in this position. Leakage of gas from the second chamber reduces the operating pressures of the system and may substantially impair the ability of the accumulator to provide hydraulic fluid at a desired pressure to the hydraulic system when the demand of the hydraulic system increases.
Furthermore, in some applications, process systems may be located in remote locations such as, for example, off-shore drilling wells, mining operations, oil fields, etc. Such remote locations make it difficult and costly to access accumulator apparatus for maintenance and/or to re-charge the accumulator apparatus with a gas. Also, having to charge accumulator apparatus with a fluid significantly increases maintenance costs.